Ocean circulation and shelf processes in the Arctic Mediterranean traced by radiogenic neodymium isotopes, rare earth elements and stable oxygen isotopes

The warming of the Arctic region in the recent past has proceeded at rates double that of the global average and has been accompanied by rapid sea ice retreat and increased heat and freshwater fluxes to the Arctic Mediterranean (i.e. the Arctic Ocean and the Nordic Seas, AM). Further warming will have strong impacts on ocean circulation, freshwater pathways, and marine ecosystems in the AM. Disentangling the sources, distribution and mixing of water masses involved in the transport and transfer of heat and freshwater is therefore critical for the understanding of present and future hydrological changes in the high-latitude and polar regions and their consequences. This study refines the knowledge of water mass circulation and mixing in the AM and provides new insights into the processes occurring on the Arctic shelves and in high-latitude estuaries. A multi-proxy approach is used combining dissolved radiogenic Nd isotopes (εNd), rare earth elements (REEs) and stable oxygen isotopes (δ18O) together with standard hydrographic tracers. The sources, distribution and mixing of water masses that circulate in the AM and pass the Fram Strait are assessed through evaluation of dissolved εNd and REE, and δ18O data obtained from samples recovered in 2012 along a full water depth section extending between Svalbard and Greenland at ~79 °N, and through a compilation and reassessment of literature Nd isotope and concentration data previously reported for other sites within the AM. The Nd isotope and REE distribution in the central Fram Strait and the open AM primarily reflects the lateral advection of water masses and their mixing, whereas seawater-particle interactions exert important control only above the shelf regions. For example, on the NE Greenland Shelf, remineralization of biogenic and/or release from detrital particles is recorded in bottom waters. Advection of warm Atlantic Water (AW) in the upper water column of the eastern and central Fram Strait is clearly reflected by an εNd signature of -11.7 and a Nd concentration ([Nd]) of 16 pmol/kg. Freshening and cooling of the AW on its way through the AM are accompanied by a continuous change towards more radiogenic Nd isotope compositions (e.g. -10.4 of dense Arctic Atlantic Water). This change results from mixing with intermediate waters but also mirrors the admixture of dense Kara Sea waters and Pacific-derived waters. Exchange with basaltic formations of Iceland and southeastern Greenland is suggested to impart the intermediate and deep waters of the AM with more radiogenic εNd signatures, which reach -9.5 in the Fram Strait. Significant inputs of Nd from Svalbard are not observed and surface waters and Nd on the western Svalbard Shelf originate in the Barents Sea. Shallow (< 200 m) waters of Arctic origin form the core of the East Greenland Current above the Greenland slope and have relatively radiogenic εNd (reaching -8.8) and elevated [Nd] (21-29 pmol/kg), which together with δ18O and standard hydrographic tracers are used to determine the proportions of Pacific-derived (< 30 % based on Nd isotopes) and Atlantic-derived seawater, as well as of river waters (< 8 %). A change in the Nd isotope compositions to less radiogenic values (-12.4) and an increase in [Nd] (38 pmol/kg) are observed at water depths above 100 m near the Greenland coast documenting addition of Greenland-sourced freshwater (GFW). The amount of GFW contained in the upper water column on the NE Greenland Shelf reached 6 % in 2012. Data obtained for the years 2014 and 2015 for the northern and southern NE Greenland Shelf suggest similar fractions of GFW for shallow waters in the Norske Trough and east of Ob Bank, indicating southward and northward propagation of GFW along the Greenland coast assuming that the NE Greenland Ice Stream is the freshwater source. The Nd isotope compositions of Arctic-derived waters (εNd ~ -9) and other water masses were essentially constant over the time period 2012-2015, which provides a solid basis for quantitative estimates of GFW admixture. The GFW distribution suggests that future increased GFW supply forced by global warming will lead to additional freshening of shallow Arctic waters, which, once these waters have traversed the Nordic Seas, may ultimately affect overturning strength in the northern Labrador Sea. Overall, the results obtained from the Fram Strait demonstrate that the pronounced gradients in εNd and REE contents in the upper water column provide a reliable basis for assessments of short-term shallow hydrological changes within the AM. New insights into the processes occurring in high latitude estuaries are provided by dissolved Nd isotope and REE compositions together with δ18O data for the Laptev Sea based on filtered samples collected during two summers (2013 + 2014) and one winter (2012). The Laptev Sea is a Siberian Shelf sea characterized by extensive river-runoff, sea-ice production and ice transport into the Arctic Ocean. The broad range in εNd (-6 to -17), REE concentrations (16 to 600 pmol/kg for Nd) and REE patterns found in the frame of this study is attributed to freshwater supply from the Siberian rivers and advection of open ocean Arctic Atlantic Water. Strikingly and contrary to expectations, there is no evidence for significant release of Nd from particulate phases and Nd isotopes can thus be used to assess water mass mixing together with the salinity after correction for variations in the salinity caused by sea-ice formation and melting. High fractions of riverine contributions from the Lena River (up to 75 %) are determined for the surface layer of the eastern Laptev Sea with significant interannual variations, while the less variable advection of Yenisei and Ob freshwater (up to ~20 %) is restricted to the western Laptev Sea. Essentially all Laptev shelf waters are depleted in light (L)REEs, while the distribution of the heavy (H)REEs shows a deficiency at the surface and an excess in the bottom layer. A combination of REE removal through coagulation of nanoparticles and colloids and REE redistribution within the water column through formation and melting of sea ice and river ice is suggested to account for the distribution of all REEs. Estuarine removal of riverine REEs starts at salinities close to 10 and after a drop of all REEs by about 30 % transfers into preferential LREE removal, which for Nd reaches 75 % at salinities near 34. Although the delayed onset of dissolved REE removal contrasts with observations from most other estuarine environments, the distributions coincide remarkably well with results from recent experiments simulating estuarine mixing with organic-rich river waters. The melting and formation of sea ice and river ice lead to further REE depletion at the surface and enrichments in the bottom water layer as a function of ice melting and brine transfer, respectively. The ice-related processes contribute to the redistribution of other elements and may also affect macronutrient distribution and primary productivity in high latitude estuaries